High full-well capacity pixel with graded photodetector implant

ABSTRACT

Embodiments of a process for forming a photodetector region in a CMOS pixel by dopant implantation, the process comprising masking a photodetector area of a surface of a substrate for formation of the photodetector region, positioning the substrate at a plurality of twist angles, and at each of the plurality of twist angles, directing dopants at the photodetector area at a selected tilt angle. Embodiments of a CMOS pixel comprising a photodetector region formed in a substrate, the photodetector region comprising overlapping first and second dopant implants, wherein the overlap region has a different dopant concentration than the non-overlapping parts of the first and second implants, a floating diffusion formed in the substrate, and a transfer gate formed on the substrate between the photodetector and the transfer gate. Other embodiments are disclosed and claimed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/755,088, filed 6 Apr. 2010, and claims priority therefrom under 35U.S.C. §120. U.S. application Ser. No. 12/755,088 is currently pendingand is hereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the present invention relate to semiconductor pixels andin particular, but not exclusively, to a pixel with a gradedphotodetector implant having a high full-well capacity.

BACKGROUND

As the pixel size of complementary metal oxide semiconductor (CMOS)image sensors becomes smaller for higher pixel density and lower cost,the active area of the photodetector also becomes smaller. For pinnedphotodetectors that are commonly used in CMOS image sensors, the smallerphotodetector area leads to reduced full-well capacity, meaning that themaximum number of charges that can be held in the photodetector isreduced. The reduced full-well-capacity in turn results in a pixel withlower dynamic range and lower signal-to-noise ratio. Therefore, methodsto increase the full-well-capacity of the pinned photodetector arehighly desired.

In the p-n-p pinned photodetector most commonly used for CMOS imagesensors, the most straightforward way to increase the pixel's full wellcapacity is to increase the doping level (i.e., the concentration ofdopants) in the n-type layer, for example by increasing the implantationdosage. For small pixel sizes, however, the increased n-type doping canlead to significant increase in dark current and in defective pixelscommonly referred to as white pixels. One reason for this is because ofthe increased electrical field along shallow trench isolation (STI)sidewalls due to the high n-type doping and the shrinking distancebetween n-type implant and STI edge.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified. Figures are not to scale unless specificallydesignated as being to scale.

FIG. 1A is a plan view of an embodiment of a pixel.

FIG. 1B is a combination schematic and cross-sectional view of theembodiment of a pixel shown in FIG. 1A, the cross-section being takensubstantially along section line B-B in FIG. 1A.

FIG. 1C is a cross-sectional view of the embodiment of the pixel shownin FIGS. 1A-1B, taken substantially along section line C-C in FIG. 1A.

FIG. 1D is a cross-sectional elevation of an alternative embodiment of apixel.

FIG. 2 is an isometric drawing of a spherical coordinate systemsuperimposed on a Cartesian coordinate system.

FIGS. 3A-3B are, respectively, a cross-sectional elevation and a planview illustrating an embodiment of a process for manufacturing theembodiments of a pixel shown in FIGS. 1A-1D.

FIGS. 4A-4C are, respectively, a plan view, a cross-sectional viewsubstantially along section line 4B-4B of FIG. 4A, and a cross-sectionalview substantially along section line 4C-4C of FIG. 4A furtherillustrating an embodiment of a process for manufacturing theembodiments of a pixel shown in FIGS. 1A-1D.

FIGS. 5A-5C are, respectively, a plan view, a cross-sectional viewsubstantially along section line 5B-5B of FIG. 5A, and a cross-sectionalview substantially along section line 5C-5C of FIG. 5A furtherillustrating an embodiment of a process for manufacturing theembodiments of a pixel shown in FIGS. 1A-1D.

FIGS. 6A-6B are cross-sectional views further illustrating an embodimentof a process for manufacturing the embodiments of a pixel shown in FIGS.1A-1D.

FIG. 7 is a block diagram of an embodiment of an imaging system that canemploy an embodiment of an image sensor using the pixel embodimentsshown in FIG. 1A-1D.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Embodiments of apparatus, system and method for a pixel with a gradedimplant having a high full-well capacity are described herein. In thefollowing description, numerous specific details are described toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventioncan be practiced without one or more of the specific details, or withother methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail but are nonetheless encompassed within the scope ofthe invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in thisspecification do not necessarily all refer to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

FIGS. 1A-1C together illustrate an embodiment of a complementary metaloxide semiconductor (CMOS) pixel 100, such as those found in a pixelarray within an image sensor. FIG. 1A illustrates a planar view of pixel100. FIG. 1B illustrates a cross-sectional view of a portion of pixel100, taken substantially along section line B-B in FIG. 1A. FIG. 1Cillustrates a cross sectional view of a portion of pixel 100 takensubstantially along section line C-C in FIG. 1A. Illustrated pixel 100is an active four-transistor pixel, commonly known as a “4T activepixel,” but in other embodiments pixel 100 could include more or lesstransistors. As shown in FIG. 1B pixel 100 is formed in an epitaxial(epi) layer 104 formed on substrate 102, and includes a photodetector106, a floating node 114, and a transfer gate 112 that, when switchedon, transfers charge accumulated in photodetector 106 to floating node114. Shallow trench isolations (STIs) 116, 124 and 126 can be used tophysically separate and electrically isolate pixel 100 from adjacentpixels in a pixel array. FIG. 1A shows a plan view of pixel 100 with STIregions shown as regions 116, 124 and 126, floating node 114 andtransfer gate 112.

As shown in FIG. 1B, photodetector 106 is formed in epi layer 104 andincludes a pinning region 110 and an implant 108 abutting and at leastpartially surrounding pinning region 110. In the illustrated embodiment,pinning region 110 is positioned at or near the surface of epi layer104, but in other embodiments the pinning region can be positionedelsewhere or can be omitted entirely. In the illustrated embodimentpinning region 110 is a P-type region, implant 108 forms an N-typeregion and epi layer 104 is a P-type region, making photodetector 106 ap-n-p photodetector. In other embodiments of photodetector 106 thecharge types (e.g, positive or negative charge carriers) of theseelements can be reversed—that is, in other embodiments pinning region110 can be N-type, implant region 108 can be P-type and epi layer 104can be N-type, forming an n-p-n photodetector. In still otherembodiments epi layer 104 can be undoped, whatever the charge types ofthe pinning region and the implant.

As shown in FIGS. 1A and 1C, in the illustrated embodiment ofphotodetector 106 implant region 108 includes three different componentregions: an overlap region 108 c and a pair of non-overlap regions 108 aand 108 b. Overlap region 108 c, so called because it results from theoverlap of two or more implanted regions, has a relatively higher dopantconcentration than non-overlap regions 108 a and 108 b. As a resultimplant region 108 can have a larger lateral extent (i.e., it occupiesmore of the space between STIs 124 and 126 or, put differently, has asmaller distance d₁ and d₃) and is graded (i.e., it has a spatial dopantconcentration gradient). Combining larger lateral extent with gradingresults in an implant with high full-well capacity but less of theproblems associated with high electric fields at the lateral edges ofthe implant. As explained below, implant region 108 has three differentcomponent regions because it is made by overlapping two implant regions.

In a p-n-p embodiment of pixel 100, during an integration period (alsoreferred to as an exposure period or accumulation period) photodetector106 receives incident light, as shown by the arrow in FIG. 1B, andgenerates charge at the interface between pinning region 110 and implantregion 108. After the charge is generated it is held as free electronsin implant region 108. At the end of the integration period, theelectrons held in N-type implant region 108 (i.e., the signal) aretransferred into floating node 114 by applying a voltage pulse to turnon transfer gate 112. When the signal has been transferred to floatingnode 114, transfer gate 112 is turned off again for the start of anotherintegration period of photodetector 106. After the signal has beentransferred from N-type implant region 108 to floating node 114, thesignal held in floating node 114 is used to modulate amplificationtransistor 124, which is also known as a source-follower transistor.Finally, address transistor 122 is used to address the pixel and toselectively read out the signal onto the signal line. After readoutthrough the signal line, a reset transistor 120 resets floating node 114to a reference voltage, which in one embodiment is V_(dd).

FIG. 1D illustrates a cross-section of an alternative embodiment of apixel 150. Pixel 150 is in most respects similar to pixel 100, theprimary difference being the structure of the implanted region. In pixel150, the photodetector also includes a graded implant region 152 withthree component regions: an overlap region 152 c with a relativelyhigher dopant concentration and two non-overlap regions 152 a and 152 bwith relatively lower dopant concentrations. Unlike implant region 108,in implant region 152 non-overlap regions 152 a and 152 b includenotches 154 and 156 on the sides of the implanted region. Notches 154and 156 can result from the tilt angle at which dopants are implantedinto epi layer 104 on substrate 102.

FIG. 2 illustrates a spherical coordinate system and its superpositiononto a Cartesian coordinate system. A spherical coordinate system isdefined by (i) a reference plane containing an origin and an azimuthreference direction, and (ii) a zenith, which is a line that passesthrough the origin and is normal to the reference plane. In FIG. 2, theorigin O is formed at the intersection of the x, y and z axes, Cartesianx-y plane forms the reference plane, the x axis forms the azimuthreference direction, and the z axis forms the zenith. The sphericalcoordinates of a point P are given by its radius R; its inclinationangle θ, which is the angle between the zenith and the line segment OP;and its azimuth ψ, which is the angle measured from the azimuthreference direction to the orthogonal projection of the line segment OPon the reference plane (the x-y plane in this case). In FIG. 2, angle αis the angle relative to the zenith (the z axis in this case) of theorthogonal projection of OP onto the x-z plane, and angle β is the anglerelative to the zenith (the z axis in this case) of the orthogonalprojection of OP onto the y-z plane.

FIGS. 3A-3B together illustrate an initial part of an embodiment of aprocess for forming the pixel embodiments shown in FIGS. 1A-1D; FIG. 3Bis a plan view, while FIG. 3A illustrates a cross-sectional view takensubstantially along section line 3A-3A in FIG. 3B. In the illustratedembodiment, shown in FIG. 3A, elements of pixel 100 other thanphotodetector 106 (i.e., STIs 116, 124 and 126, floating diffusion 114,transfer gate 112, and so forth) are first formed in epi layer 104 onsubstrate 102, after which a mask layer 302 is applied to the front sideof epi layer 104. Mask layer 302 is designed to prevent dopants fromimpinging on and penetrating epi layer 104 during dopant implantation.In one embodiment, mask layer 302 can be made of conventionalphotoresist, but in other embodiments it can be of a differentsubstance. Mask layer 302 can be applied by various known methods.

After mask layer 302 is in place, it is patterned and etched by knownmethods (such as photolithography and wet or dry chemical etching) tocreate an opening 304 of width W in the mask layer, exposing the frontside of a region of epi layer 104 in which photodetector 106 will beformed. With opening 304 in mask layer 302, dopants can be implanted inthe desired region without also implanting them in other regions whereother components are or will be formed. The illustrated embodiment onlyshows that part of mask layer 302 that surrounds opening 304 and showsother elements of pixel 100 exposed. While such an arrangement can beused in one embodiment, in other embodiments the mask layer can coverall or some of the other illustrated pixel elements, such as floatingnode 114, gate 112, etc., during dopant implantation.

Substrate 102 is positioned such that is can be twisted about an axis306 that is substantially normal to the front side of substrate 102. Interms of the spherical coordinate system shown in FIG. 2, substrate 102and the elements formed on it are positioned substantially in thereference plane (the x-y plane in FIG. 2), and axis 306 corresponds tothe zenith (the z axis in FIG. 2). In the illustrated embodiment, axis306 coincides approximately with the center of opening 304, but in otherembodiments axis 306 can be offset from opening 304. For example, in aproduction environment there are usually many image sensors, each with apixel array having a large number of pixels, formed on a semiconductorwafer. In such a production environment, axis 306 can coincide with thecenter of the semiconductor wafer, which may or may not coincide withthe center of any pixel or pixel array on the wafer.

As shown in FIG. 3B, substrate 102 can be twisted about axis 306 to anyarbitrary twist angle ψ relative to a reference direction. In theillustrated embodiment, twist angle ψ is defined as the angle between afixed reference line 310 and a line 308 that rotates with the substrateand substantially bisects opening 304 and floating diffusion 114. Withreference again to FIG. 2, reference line 310 is analogous to theazimuth reference (the x axis in FIG. 2), and twist angle ψ is analogousto the azimuth angle. In other embodiments twist angle ψ can be defineddifferently, so long as it can be used to characterize a rotation ofsubstrate 102 about an axis such as axis 306.

FIGS. 4A-4C illustrate another part of an embodiment of a process forforming the photodetector implant region in the pixel embodiments shownin FIGS. 1A-1D. FIG. 4A is a plan view, while FIGS. 4B and 4C illustratecross-sectional views taken substantially along section lines 4B-4B and4C-4C in FIG. 4A. With ψ as shown in FIG. 3B, the initial twist positioncorresponds to ψ=a°.

As shown in FIG. 4B, after positioning the substrate at the initialtwist position, dopants 410 are directed at the exposed surface of epilayer 104. In addition to being directed at the substrate at a selecteddosage and energy, dopants 410 are directed at the substrate at anon-zero tilt angle θ relative to a line substantially normal to thefront side of epi layer 104, in this case axis 306. Again referring toFIG. 2, with substrate 102 and the elements formed on it positionedsubstantially in the reference plane (the x-y plane in FIG. 2), tiltangle θ corresponds to inclination angle θ shown in FIG. 2. Because ofthe presence of mask layer 302, dopants 410 are only able to reach theexposed front side of epi layer 104 in opening 304. As further discussedbelow, tilt angle θ is selected based on various factors including thelateral extent desired for implant region 406 under mask 302.

As dopants 410 bombard the exposed part of the front side of epi layer104, they penetrate into the interior of epi layer 104 and form a firstimplant region 406 within epi layer 104. As a result of the non-zerotilt angle θ, a portion 408 of first implant 406 ends up being formed inthe part of epi layer 104 that is underneath mask layer 302 (see alsoFIG. 4C). Another portion 409 of first implant region 406 is formed dueto twist angle ψ=a° in the part of epi layer 104 that is underneathtransfer gate 112. Use of tilt angle θ thus increases the lateral extentof the implant and reduces the distance between implant region 406 andSTI 124 (i.e., distance d₁ in FIG. 1C). Generally, the larger the tiltangle θ, the more first implant region 406 will extend laterally, makingportion 409 larger and also making region 408 under mask layer 302larger and distance d₁ between the edge of implant region 406 and STI124 smaller.

FIG. 4C illustrates how portion 408 of implant 406 extends under masklayer 302. Since the plane along which section 4C-4C is at an anglerelative to the plane along which section 4B-4B is taken, the angle αshown in FIG. 4C is not tilt angle θ, but instead is the orthogonalprojection of tilt angle θ onto the plane of section 4C-4C.

FIGS. 5A-5C together illustrate another part of an embodiment of aprocess for forming the photodetector implant region in the pixelembodiments shown in FIGS. 1A-1D. FIG. 5A is a plan view, while FIGS. 5Band 5C illustrate cross-sectional views taken substantially alongsection line 5B-5B and 5C-5C in FIG. 5B. Starting with the state shownin FIGS. 4A-4B, substrate 102 is rotated about axis 306, as illustratedby arrow 402, to an additional twist position different than the initialtwist position. With ψ defined as shown in FIG. 3B, the additional twistposition illustrated in FIG. 5B corresponds to ψ=−a°, which could beinterpreted as a reflection of line 308 about fixed line 310. Althoughonly one additional twist position is illustrated, in other embodimentsthere can be more than one additional twist position after the initialtwist position, depending on the desired final structure ofphotodetector implant region 108.

After positioning substrate 102 at the additional twist position,dopants 410 are again directed at the front surface of epi layer 104. Inaddition to being directed at the substrate with a selected dosage andenergy, dopants 410 are again directed at the substrate at a non-zerotilt angle θ relative to a line substantially normal to the front sideof epi layer 104, in this case axis 306. As before, tilt angle θ isselected based on various factors including the desired lateral extentof the implant region. In one embodiment, the tilt angle θ used at theadditional twist position can be the same as the tilt angle used at theinitial twist position, but in other embodiments the tilt angle used atan additional twist position need not be the same as the tilt angle usedat the initial twist position or the tilt angle used at any otheradditional twist position.

Because of the presence of mask layer 302, dopants 410 are only able toreach the exposed front side of epi layer 104 in opening 304. As dopants410 bombard the exposed part of the front side of epi layer 104, theypenetrate the surface and form a second implant 502 within the epilayer. Second implant region 502 overlaps in part with first implantregion 406 to form an overlap region 503 with relatively higher dopantconcentration. The combination of first implant region 406 with secondimplant region 502 thus creates photodetector implant region 108 ofFIGS. 1A to 1D, in which overlap region 108 c corresponds to overlapregion 503, non-overlapping region 108 b corresponds to the region offirst implant 406 that does not overlap with second implant region 502,and non-overlapping region 108 a of FIGS. 1A and 1C corresponds to theregion of second implant 502 that does not overlap with first implantregion 406. As a result of the overlap of first implant region 406 andsecond implant region 502, overlap region 108 c has a relatively higherdopant concentration than non-overlapping regions 108 a and 108 b,making implant 108 a graded implant.

As with first implant region 406, the non-zero tilt angle θ at whichdopants are implanted results in a portion 504 of second implant region502 being formed in the part of epi layer 104 that is underneath masklayer 302. Another portion 505 of second implant region 502 is formeddue to twist angle ψ=−a°, in the part of epi layer 104 that isunderneath transfer gate 112. Use of tilt angle θ thus increases thelateral extent of second implant region 502 and reduces the distancebetween implant region 502 and STI 126 (i.e., distance d₃ in FIG. 1C).Generally, the larger the tilt angle θ, the more second implant region502 will extend laterally, making region 504 under mask layer 302 largerand the distance d₃ between the edge of implant region 502 and STI 126smaller. As seen in FIG. 5A, the overlap of portion 409 of first implantregion 406 and portion 505 of second implant region 502 forms overlapportion 506 under transfer gate 112.

FIG. 5C illustrates a cross-section of implant 406 taken along line5C-5C in FIG. 5A, and shows how portions 408 and 504 extend under masklayer 302. Since the plane along which section 5C-5C is at an anglerelative to the plane along which section 5B-5B is taken, the angle αshown in FIG. 5C is not tilt angle θ, but instead is the orthogonalprojection of tilt angle θ onto the plane of section 5C-5C.

FIGS. 6A-6B together illustrate another part of an embodiment of aprocess for forming the photodetector implant region in the pixelembodiments shown in FIGS. 1A-1D. In FIG. 6A, photodetector implantregion 108 has been formed in epi layer 104 and mask layer 302 remainsin place on the front side of epi layer 104. In FIG. 6B, mask layer 302is removed from the front side of substrate 102, leaving graded implantregion 108 with one lateral side at a distance d₁ from STI 124 and theopposite lateral side at a distance d₃ from STI 126. Because implantregion 108 is graded, distances d₁ and d₃ can be smaller than they wouldotherwise need to be to avoid creating problems associated with highelectric fields at the edges of the implant, such as dark current andwhite pixels.

In other embodiments, mask layer 302 can be left in place while otherelements of pixel 100, such as pinning layer 110, are formed; mask layer302 can then be removed later to leave the final pixel 100.Alternatively, if any more pixel elements remain to be formed, masklayer 302 can be removed and additional elements added to pixel 100 withor without the use of additional mask layers.

FIG. 7 illustrates an embodiment of an imaging system 700. Optics 701,which can include refractive, diffractive or reflective optics orcombinations of these, are coupled to image sensor 702 to focus an imageonto the pixels in pixel array 704 of the image sensor. Pixel array 704captures the image and the remainder of imaging system 700 processes thepixel data from the image.

Image sensor 702 comprises a pixel array 704 and a signal reading andprocessing circuit 710. In one embodiment, image sensor 702 is abackside-illuminated image sensor including a pixel array 704 that istwo-dimensional and includes a plurality of pixels arranged in rows 706and columns 708. One or more of the pixels in pixel array 704 can be oneof the pixel embodiments shown in FIGS. 1A-1D. During operation of pixelarray 704 to capture an image, each pixel in pixel array 704 capturesincident light (i.e., photons) during a certain exposure period andconverts the collected photons into an electrical charge. The electricalcharge generated by each pixel can be read out as an analog signal, anda characteristic of the analog signal such as its charge, voltage orcurrent will be representative of the intensity of light that wasincident on the pixel during the exposure period.

Illustrated pixel array 704 is regularly shaped, but in otherembodiments the array can have a regular or irregular arrangementdifferent than shown and can include more or less pixels, rows, andcolumns than shown. Moreover, in different embodiments pixel array 704can be a color image sensor including red, green, and blue pixelsdesigned to capture images in the visible portion of the spectrum, orcan be a black-and-white image sensor and/or an image sensor designed tocapture images in the invisible portion of the spectrum, such asinfra-red or ultraviolet.

Image sensor 702 includes signal reading and processing circuit 710.Among other things, circuit 710 can include circuitry and logic thatmethodically reads analog signals from each pixel, filters thesesignals, corrects for defective pixels, and so forth. In an embodimentwhere circuit 710 performs only some reading and processing functions,the remainder of the functions can be performed by one or more othercomponents such as signal conditioner 712 or DSP 716. Although shown inthe drawing as an element separate from pixel array 704, in someembodiments reading and processing circuit 710 can be integrated withpixel array 704 on the same substrate or can comprise circuitry andlogic embedded within the pixel array. In other embodiments, however,reading and processing circuit 710 can be an element external to pixelarray 704 as shown in the drawing. In still other embodiments, readingand processing circuit 710 can be an element not only external to pixelarray 704, but also external to image sensor 702.

Signal conditioner 712 is coupled to image sensor 702 to receive andcondition analog signals from pixel array 704 and reading and processingcircuit 710. In different embodiments, signal conditioner 712 caninclude various components for conditioning analog signals. Examples ofcomponents that can be found in the signal conditioner include filters,amplifiers, offset circuits, automatic gain control, etc. In anembodiment where signal conditioner 712 includes only some of theseelements and performs only some conditioning functions, the remainingfunctions can be performed by one or more other components such ascircuit 710 or DSP 716. Analog-to-digital converter (ADC) 714 is coupledto signal conditioner 712 to receive conditioned analog signalscorresponding to each pixel in pixel array 704 from signal conditioner712 and convert these analog signals into digital values.

Digital signal processor (DSP) 716 is coupled to analog-to-digitalconverter 714 to receive digitized pixel data from ADC 714 and processthe digital data to produce a final digital image. DSP 716 can include aprocessor and an internal memory in which it can store and retrievedata. After the image is processed by DSP 716, it can be output to oneor both of a storage unit 718 such as a flash memory or an optical ormagnetic storage unit and a display unit 720 such as an LCD screen.

The above description of illustrated embodiments of the invention,including what is described in the abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. These modifications can bemade to the invention in light of the above detailed description.

The terms used in the following claims should not be construed to limitthe invention to the specific embodiments disclosed in the specificationand the claims. Rather, the scope of the invention is to be determinedentirely by the following claims, which are to be construed inaccordance with established doctrines of claim interpretation.

The invention claimed is:
 1. A CMOS pixel comprising: a photodetectorregion formed in a substrate by dopant implantation, the photodetectorregion comprising overlapping first and second dopant implants, whereinthe overlap region has a different dopant concentration than thenon-overlapping parts of the first and second dopant implants, whereinthe first dopant implant is formed by directing dopants at the substrateat a selected tilt angle and at an initial twist position and the seconddopant implant is formed by directing dopants at the substrate at theselected tilt angle and at an additional twist position different thanthe initial twist position, and wherein the twist position is an angleof rotation about an axis normal to the surface of the substrate; afloating diffusion formed in the substrate; and a transfer gate formedon the substrate between the photodetector and the transfer gate.
 2. TheCMOS pixel of claim 1 wherein the photodetector region includes a notchin each of two opposing lateral sides thereof.
 3. The CMOS pixel ofclaim 1 wherein the tilt angle is measured relative to a linesubstantially normal to the surface of the substrate.
 4. The CMOS pixelof claim 1, further comprising a pinning layer with a charge typeopposite the charge type of the photodetector region, wherein thepinning layer is formed over the photodetector region and at or near thesurface of the substrate.
 5. The CMOS pixel of claim 1 wherein thenon-overlapping part of the first dopant implant has a different dopantconcentration than the non-overlapping part of the second dopantimplant.
 6. A system comprising: a CMOS image sensor formed in asubstrate, wherein the CMOS image sensor has a pixel array including aCMOS pixel comprising: a photodetector region formed in a substrate bydopant implantation, the photodetector region comprising overlappingfirst and second dopant implants, wherein the overlap region has adifferent dopant concentration than the non-overlapping parts of thefirst and second dopant implants, wherein the first dopant implant isformed by directing dopants at the substrate at a selected tilt angleand at an initial twist position and the second dopant implant is formedby directing dopants at the substrate at the selected tilt angle and atan additional twist position different than the initial twist position,and wherein the twist position is an angle of rotation about an axisnormal to the surface of the substrate, a floating diffusion formed inthe substrate, and a transfer gate formed on the substrate between thephotodetector and the transfer gate; and processing circuitry coupled tothe pixel array to process a signal received from the pixel array. 7.The system of claim 6 wherein the photodetector region includes a notchin each of two opposing lateral sides thereof.
 8. The system of claim 6wherein the tilt angle is measured relative to a line substantiallynormal to the surface of the substrate.
 9. The system of claim 6 whereinthe CMOS pixel further comprises a pinning layer with a charge typeopposite the charge type of the photodetector region, wherein thepinning layer is formed over the photodetector region and at or near thesurface of the substrate.
 10. The system of claim 6 wherein thenon-overlapping part of the first dopant implant has a different dopantconcentration than the non-overlapping part of the second dopantimplant.
 11. The system of claim 6 wherein the processing circuitryincludes a digital signal processor coupled to the image sensor toprocess the signals received from the image sensor.